ultimaker 2 led current on pwm

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Hello, I have built my own UM2 as the aluminium version from thingiverse. I have the original UM2 Mainboard and what to install the LED Stripes to see what I am printing.I have bought 24V 5050 SMD led stripes and don't know how long the stripe could be. I want to cut the part I think and then measure the current from an external power supply. Therefore I need to know the current of the pwm. Can anyone help me please?

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Yes, the PWM is generated by the microprocessor and delivered through a BC817 transistor.

The max current for the BC817 is 500mA, but you need to stay away from that limit unless you like SMD soldering...

If only life was so simple. 500mA is the maximum power. However, the true limitation is power dissipation. As there is no active cooling, your true limit is heat based. For that you need to calculate the amount of power "spend" in the transistor, and know the thermal properties (some of which are in the datasheet, some are "defined" by the board), the maximum temperature allowed by the transistor and the ambient temperature.

I haven't done this math in 10 years, so I don't remember the exact details.

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Yes, the PWM is generated by the microprocessor and delivered through a BC817 transistor.

The max current for the BC817 is 500mA, but you need to stay away from that limit unless you like SMD soldering...

If only life was so simple. 500mA is the maximum power. However, the true limitation is power dissipation. As there is no active cooling, your true limit is heat based. For that you need to calculate the amount of power "spend" in the transistor, and know the thermal properties (some of which are in the datasheet, some are "defined" by the board), the maximum temperature allowed by the transistor and the ambient temperature.

True, but the power dissipation in the transistor should be minimal due to it being PWM driven and ideally in a fully on or fully off state at all times. In the ideal world, the transistor should be run saturated and responsible for a 0V drop collector-emitter.

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I think it might be a good idea to google the BC817 specs, and test things out with a separate transistor and a test board. If it would go wrong, that is a lot cheaper than a new controller.

The very limited specs I found (but I only did a quick search), do list a VCEsat of 700mV at 500mA, which is surprisingly high. I would have expected 300...400mV. And a Ptot of 250mW at 25°C ambient temp. So, at a load of 300mA you could still go over the max power rating in a closed environment with limited airflow and cooling (thus probably 50 to 60°C in summer).

So you need to find specs that show all the power-rating curves and VCE-curves too, for given loads and temps.

My practical experience with discrete transistors (long ago in the stone age, before SMD...) is that they don't like it when you get close to their maximums. Some even didn't like it when you only got half way.

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The very limited specs I found (but I only did a quick search), do list a VCEsat of 700mV at 500mA, which is surprisingly high. I would have expected 300...400mV. And a Ptot of 250mW at 25°C ambient temp. So, at a load of 300mA you could still go over the max power rating in a closed environment with limited airflow and cooling (thus probably 50 to 60°C in summer).

That's because you're looking at Vcesat max, and it's measured at Icm, which is the peak collector current, 1A for this device. At 300mA you're looking at Vce < 200mV

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I can only add that in my personal experience after breaking the led and fans pwm bc817 8 times with different fans the only way they survive is by not going beyond 300mA. Also fun experience to learn how to solder SMD. At least the equipment to fix it is much cheaper than a new board.

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I can only add that in my personal experience after breaking the led and fans pwm bc817 8 times with different fans the only way they survive is by not going beyond 300mA. Also fun experience to learn how to solder SMD. At least the equipment to fix it is much cheaper than a new board.

I've been meaning to ask you about that, actually. I assume you've pushed them to around the max. current rating. Was it on the same board every time?

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I can only add that in my personal experience after breaking the led and fans pwm bc817 8 times with different fans the only way they survive is by not going beyond 300mA. Also fun experience to learn how to solder SMD. At least the equipment to fix it is much cheaper than a new board.

I've been meaning to ask you about that, actually. I assume you've pushed them to around the max. current rating. Was it on the same board every time?

Same board, 350mA crossflow fan, then again the led pwm. After paying 50€ to an expert I asked a friend to teach me. After a few months, installed 2 150mA fans, fan died while at 255 and turned it off the machine. Then fixed it and did more tests on the led pwm, died too. Then tested other fans model of 0.60 while doing dual so x4 0.60mA and again fan pwn died after a few months suddenly.

Then again by doing a bad +/- led connection. Then again the fan with two 120mA fans and then again dunno how. Last time I even posted a few photos of how I did it.

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Same board, 350mA crossflow fan, then again the led pwm. After paying 50€ to an expert I asked a friend to teach me. After a few months, installed 2 150mA fans, fan died while at 255 and turned it off the machine. Then fixed it and did more tests on the led pwm, died too. Then tested other fans model of 0.60 while doing dual so x4 0.60mA and again fan pwn died after a few months suddenly.

Then again by doing a bad +/- led connection. Then again the fan with two 120mA fans and then again dunno how. Last time I even posted a few photos of how I did it.

And I realise now it was both the PWM outputs. I was about to suggest you might have a bad kickback diode that might be causing it... but that would of course mean both of them, then. Hm. Anyway, another cause of it all might be non-constant load offered by a fan. The inrush current might be a problem.

Anyway with the board I built I'm planning on doing some testing with a HEXFET equipped board very soon. I'm sure I can come up with a drop-in replacement part with performance far superior to the BC817. Just need to measure the PWM frequency from the main board again since I conveniently wrote it down and then apparently discarded the note.

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